There are two types of fluid-pressure-driven actuators which employ elastomeric (rubbery) shells (often called "bladders"), namely: (a) twistor actuators (herein often called "twistors") and (b) tension actuators (often called "linear actuators").
All such prior actuators have involved a network of relatively inextensible or non-stretchable flexible reinforcing strands in order to constrain the rubbery bladder so as to cause PG,4 the repeatedly inflated and deflated bladder to perform in a predictable manner and to prevent unpredictable localized stretching and bulging of the bladder wall. In some prior actuators, this network of inextensible strands surrounded the bladder without being bonded to the bladder. Paynter U.S. Pat. No. 4,721,030 and Kukolj U.S. Pat. No. 4,733,603 (FIG. 1) disclosed a surrounding network of inextensible strands neither bonded nor attached to the exterior surface of the bladder. In other prior actuators, this network of inextensible strands was bonded or attached to the outside surface of the bladder. Yarlott U.S. Pat. No. 3,645,173 and Paynter U.S. Pat. Nos. 4,108,050, 4,751,868 and 4,751,869 disclosed relatively inextensible strands bonded to the exterior surface of the bladder. Kukolj in FIG. 16 showed a network of non-stretchable flexible elements, such as braided wire, embedded within the elastomeric material from which the wall of the bladder was made.
It is noted that in Paynter '030 the multiple inextensible strands, which formed a network surrounding the bladder of the tension actuator shown in that patent, had the configuration of a hyperboloid of revolution when the actuator was in its deflated and elongated condition. Such configuration of the network of inextensible strands prevented any twist motion of that tension actuator.
Such prior actuators having the above-described composite structures of an elastomeric bladder with a network of relatively inextensible strands were complicated and expensive to fabricate. Considerable hand labor often was required in their fabrication. Also, the incompatibility of the non-stretchable network in association with the stretchable bladder created undesirable stress-concentration points in the bladder wall as the bladder was repeatedly inflated and deflated during multiple cycles of operation. Such stress-concentration points often caused premature tearing or rupture of the bladder wall, thereby on average leading to relatively short operating lives.
In those cases where the network of non-stretchable strands surrounded the bladder without being bonded thereto, a localized frictional, abrasive rubbing action often occurred between the non-attached strands and the bladder wall during inflation and deflation. Such abrasive rubbing action often induced premature failure, thereby causing relatively short average operating lives. It is interesting to note that Kukolj in FIGS. 11 and 13 showed a perforated, friction-reducing layer which was interposed, as shown in FIG. 9, between the inner bladder shown in FIG. 12 and an outer network of inextensible strands. In order further to reduce the deleterious effects of abrasive friction, Kukolj taught that a lubricant is applied between the friction-reducing layer and the inner bladder. It is submitted that his willingness to employ a complex, expensive, friction-reducing layer plus lubricant in such a composite structure serves to emphasize the novelty and non-obviousness of the present all-elastomer twistors.
In summary, such prior composite-structure, fluid-driven actuators were difficult to fabricate, complex and expensive and provided relatively short average operating lives.